This level is basically about surviving the Covenants that take over the battle ship, and escaping from it. You will start the game with an overview tutorial, and the gameplay start right away. At first you will need to go and meet Keyes.

You will be asked to look around, just move your mouse around to proceed. You will notice that the health bar is starting to appear. Now climb out of the Cyro-Tube. After climbing out, walk into the red platform just next to the man in the yellow costume. Next you will be asked to look into the target lights to test aim calibration, after that you will be asked to use an inverted control or a normal one. Now get to the yellow marked platform to recharge the energy shield. Just look at the man when he asks, and wait until it is over.

Captain Keyes will ask to bring in the Master Chief through the radio, so you will skip the weapon diagnosis. Do not worry, you will be given a weapon soon. An EliteCovenant will come and kill the man in the control room, proceed to the door in front of you, turn right into a small hallway where a door will explode and the man will die. Ignore him now, and find your way through the pipes at the right of the hallway. Be careful of exploding doors and walls. After reaching a checkpoint wait until the blast doors close, to avoid any damage from Covenant shootouts. Just find your way to Captain Keyes now, avoid any combat. After reaching another combat area, a trooper will ask you to fallow him to Keyes.

Get off the Control bridge, and proceed to the only door with the green light let. Next you will face three Grunts, take them out quickly. After that proceed to the cafeteria and pick up the Assault Rifle from the fallen trooper, assist the troopers in the cafeteria and take down the Covenants. Then, get out of the cafeteria and shoot the two grunts and the Elite. You can swap your assault for the plasma pistol in the cafeteria.
Proceed until you reach an airlock with a "31" marked on the wall, a couple of grunts and some elites will emerge, shoot them. You can enter the Covenant little attack ship at the right, and pickup the Overshield which acts as a third layer shield over the energy shield. Now proceed with the marines until you reach the dark hallway marked with "stairs", there is a grunts in. after that continue marching until you can see other marine troops, turn right just before them and finish off the group of grunts and the Elite. If you do so, you should take them in no time by flanking them, or you can join the marines and take cover instead. After finishing the shootout make your way to the hallway that leads to the airlock. And make your way through.

After reaching another crossway, you can join the marines or flank the grunts and the Elite again. Move on until you reach another airlock marked with "32", shoot the hostile Covenant troops. Swap weapons from the dead bodies whenever necessary.

Airlock #32

Proceed to the next area, you will find a group of Grunts and Elites, you can perform melee attacks to Grunts to save ammo. Next go to the hallway that leads to the stairs, where Cortana starts to talk to you about the Covenants upstairs. Stay down and try to finish off the Elites first, then take out the Grunts. Swap your weapons for ammo when done. Go upstairs and make your through until you can see another Covenant attack group, you can see them from a distance and they won't move, take advantage of this and shoot the Elite first. Finish off the remaining Grunts and make through the dark corner and into another Covenant group, finish them off and proceed. Make through another dark corner, use flashlight if you wish to, and make it to the lifeboat launch area.
When you reach the airlocks hallway you will find three Grunts, just shoot them or perform melee attacks to save ammo if you are short on it. Proceed to the NAV Hud point on the screen, now you will have to walk in a dark mini maze, just use your flashlight. Some Grunts are in this maze. Turn left from where you are when you just entered, and turn right to see a green-let door, when out perform a melee attack on the damaged door.
After getting in make your way through and perform a melee attack on the Grunt facing the side, it will be a one hit kill. Keep moving to find another group of Grunts and a couple of Elites, take them out and Swap weapons. Head over the hallway, and keep moving forward, you will eventually see an Elite, kill him. Turn left where the explosion took place and get in the dark maze, use flashlight to navigate. After finding your way out assist the marines to take out another group of Covenants. Keep moving and you will encounter another two separate groups, take them fast as well. Take some grenades and head over the last lifeboat. The level is over, now you will crash in Installation 04 in the next mission Halo.